I'm using "Closure Compiler", when compiling my scripts I spend the following:
Before compiling:
// ==ClosureCompiler==
// @compilation_level SIMPLE_OPTIMIZATIONS
// @output_file_name default.js
// @formatting pretty_print,print_input_delimiter
// ==/ClosureCompiler==
var myObj1 = (function() {
  var undefined;   //<----- declare undefined
  this.test = function(value, arg1) {
    var exp = 0;
    arg1 = arg1 == undefined ? true : arg1;  //<----- use declare undefined
    exp = (arg1) ? value * 5 :  value * 10;
    return exp;
  };
  return this;
}).call({});
var myObj2 = (function() {
  this.test = function(value, arg1) {
    var exp = 0;
    arg1 = arg1 == undefined ? true : arg1;  //<----- without declare undefined
    exp = (arg1) ? value * 5 :  value * 10;
    return exp;
  };
  return this;
}).call({});
Compiled:
// Input 0
var myObj1 = function() {
  this.test = function(b, a) {
    a = a == void 0 ? true : a;  //<-----
    var c = 0;
    return c = a ? b * 5 : b * 10
  };
  return this
}.call({}), myObj2 = function() {
  this.test = function(b, a) {
    a = a == undefined ? true : a; //<-----
    var c = 0;
    return c = a ? b * 5 : b * 10
  };
  return this
}.call({});
With this I believe the question of the use of "void 0 " and "undefined", is there any difference in the use or the two cases are well?.
Edit
if I define "var undefined" compiled with "void 0 ", if I did not define "undefined" compiled with "undedined. " then not a matter of number of characters between "undefined" and "void 0"
Edit II: performance, based on this link
IE 8:
  typeof: 228ms
  undefined: 62ms
  void 0: 57ms
Firefox 3.6:
  typeof: 10ms
  undefined: 3ms
  void 0: 3ms
Opera 11:
  typeof: 67ms
  undefined: 19ms
  void 0: 20ms
Chrome 8:
  typeof: 3ms
  undefined: 5ms
  void 0: 3ms
 
     
     
     
     
    